Re: [Qemu-devel] [RFC][PATCH 0/21] QEMU Object Model

[Paolo Bonzini <pbonzini@redhat.com>]

On 07/28/2011 05:04 PM, Anthony Liguori wrote:
> The only way I can see is to teach each device about this interface and
> then have a common bus.  That implies that you have:
>
> class GoldfishEnumerator : public Device {
>       GoldfishDevice *slots[N];

FWIW, there's no hardcoded limit in the interface, and the list of 
devices is unordered.  But that only means you should attach it with

     plug-set goldfish_tty::enumerator goldfish_enum

rather than

     plug-set goldfish_enum::slots[12] goldfish_tty

If you can confirm that, that's fine.

> };
>
> interface GoldfishDevice {
>       const char *get_name();
>       uint64_t get_mmio_base();
>       ...
> };
>
> class GoldfishNic : public Device, implements GoldfishDevice
> {
>      const char *get_name(void) {
>          return "nic";
>      }

      uint64_t mmio_base;
      uint64_t get_mmio_base() { return mmio_base; }
      uint64_t set_mmio_base(uint64_t addr) { mmio_base = addr; }

> };

And that's exactly my point.  It's a "stupid" interface full of 
getters/setters, which is what you get if you use only interface 
inheritance instead of, where appropriate, data containment.

Interfaces should be reserved for what really depends on the 
_implementation_ of the GoldfishNic, not for accessing a bunch of 
numbers.  There is no implementation-dependent detail of that kind in 
the GoldfishDevice (unlike other buses, even simple ones like I2C).

>> The PIC's view is more complicated than a Pin, and more similar to ISA.
>
> ISA is just a pin.  The ISA bus extender literally has five pins
> corresponding to the ISA IRQs 7, 6, 5, 4, 3.

ISA is many pins. :)  Goldfish looks similar (32 pins).

Paolo